Now select the "Magnetic Lasso Tool" to make a good selection of the only hairs. With the magnetic lasso tool selecting the hairs of the picture and with magnetic lasso tool it will be very simple to select the hairs.

Now create a new layer and start working on the new layer ideally it will be called layer 1.
Now make a feather effect with the selection of 5 pixels. This will make the blending of the hair colour look gradiently changing and natural and will not look an abrupt change in colours on hair.
Fill any colour into the selection.

Now change the opacity of the layer 1 to 10-20% and you will see that the colour of the hair is totally different.